Referring to FIG. 1, a conventional digital oscilloscope 10 displays a digitally reconstructed waveform 12 representing an input electrical signal that has been acquired from a unit under test 14 by means of a probe 16 interconnected to digital oscilloscope 10 by a cable 18. In operation, digital oscilloscope 10 acquires samples of the input signal at uniformly spaced time intervals and by means of an analog-to-digital converter converts the samples to quantized samples. The quantized samples are stored in a waveform memory until they are retrieved to render reconstructed waveform 12.
U.S. Pat. No. 4,495,586 of Andrews, assigned to the assignee of the present application, describes a digital oscilloscope that can acquire and display high-frequency electrical signals by storing a plurality of quantized signal samples in a waveform memory. The waveform memory is subsequently "read out" and displayed. The digital oscilloscope of Andrews acquires plural samples of the input signal for each repetition of the input signal, develops quantized values of the samples, and stores the quantized values in predetermined waveform memory address locations. A time interval measurement unit and a presettable counter serve to determine the correct waveform memory address locations corresponding to the time at which each signal sample was quantized. This operation is repeated for each repetition of the input electrical signal and is known as equivalent time sampling. In the digital oscilloscope of Andrews, the input signals are remotely probed and conducted to the oscilloscope by a cable. The input signals are amplified, sampled, and quantized by circuits located inside the mainframe of the oscilloscope.
Designers of digital oscilloscopes 10 have long sought to improve the displayed fidelity of waveforms 12 reconstructed from samples of very-high frequency input signals. The fidelity of the reconstructed waveform is a function of the speed and accuracy of the various circuits used to sample and quantize the acquired signal. High-frequency signals contain edges having a fast rate of voltage change per unit of time. Samples of such a signal must be acquired during a correspondingly narrow sample time in order to accurately capture the voltage of the signal at predetermined instants in time. The samples must then be stored at a stable value until quantization of each sample is completed. The accuracy and stability of signal sampling is dependent upon the time duration of the samples.
There have been previously known apparatus and methods for separately mounting buffer amplifiers and signal samplers in probes that are connected by a cable to a measurement instrument such as an oscilloscope. Tektronix, Inc. of Beaverton, Oreg., assignee of the present application, started manufacturing low-capacitance active probes in the 1960s as an accessory for the Model 585 oscilloscope. More recently, FET buffer probes have been available from various manufacturers including Tektronix, Inc. However, such probes usually require relatively high operating voltages if they are to accurately acquire signals having large amplitudes or DC components.
Other prior art signal sampling probes, such as the Model S-3 manufactured by the assignee of the present application, provide 1 gigahertz sampling bandwidth with only two picofarads of input capacitance. However, such sampling probes have required that a sampling strobe signal, generated by the oscilloscope, be transmitted to the sampling head by differentially driven coaxial cables. The presence of the sampling strobe signal can cause injection of undesired signals into the signal source and can produce aberrations on the sampled signal. When flexed or otherwise moved, the cables for remote sampling heads caused sampling delay variations as well as DC offset changes. Remote sampling head systems are also sensitive to burnout, and are relatively expensive to service and manufacture.